Engines in hybrid vehicles may be operated with an Atkinson cycle which provides a higher fuel economy compared to operation in an Otto cycle. When operating in an Atkinson cycle, the intake valve may be held open for a longer duration compared to the duration of intake valve opening during the Otto cycle. Due to the longer opening of the intake valve, the effective compression ratio in an Atkinson cycle is lower than the corresponding compression ratio achieved for an Otto cycle. For an Atkinson cycle, the effective expansion ratio is higher than the compression ratio, thereby providing higher fuel efficiency. As such, the power delivered by operating the engine with an Atkinson cycle may be lower than the power delivered by operating the engine with an Otto cycle. In addition, a fuel with lower octane content may be used while operating the engine with an Atkinson cycle.
Various approaches have been used to operate vehicle engines with the Atkinson cycle. In one example approach shown by Clark in U.S. Pat. No. 7,765,806, an Atkinson cycle is used in the engine of a hybrid vehicle to improve fuel efficiency. Therein, to increase power output of the Atkinson cycle engine during increased driver demand, a compressor may be used to selectively pressurize intake air and provide the desired power. An electric motor may be utilized to operate the compressor. Further, based on the desired power demand, an electric motor in the hybrid powertrain may be used to drive the vehicle and deliver the desired power for effective vehicle operation.
The inventors herein have recognized potential issues with the above mentioned approach. As one example, by using an electrically powered compressor during higher power demands, energy consumption of the engine may increase which may add to parasitic losses of engine power and/or battery power, thereby reducing engine efficiency. Further, during low state of charge conditions of the battery, the electric motor may not be effectively used for rotating the compressor and/or driving the vehicle. The inventors have also recognized that even with the increasing of the intake air supply, the power output of the engine operating with the Atkinson cycle may not be sufficient to meet the driver demand, thereby affecting vehicle performance and increasing noise vibration harshness (NVH) levels.
In one example, the issues described above may be addressed by a method for a hybrid vehicle including an engine and a motor, comprising: responsive to each of a lower than threshold torque demand, and a lower than threshold state of charge of a system battery, operating the engine using an Atkinson cycle, and responsive to a higher than threshold torque demand, operating the engine using an Otto cycle with an octane booster added to injected fuel. In this way, responsive to an increase in driver demanded torque, by injecting an octane additive to the fuel and by adjusting intake valve timing to operate the engine with an Otto cycle instead of an Atkinson cycle, the desired power may be delivered.
As one example, in a hybrid vehicle, during conditions when power desired for vehicle operation is lower than a threshold, and an electric motor cannot be used to deliver the desired power, such as due a battery state of charge being lower than a threshold, the vehicle engine may be operated with an Atkinson cycle to deliver the desired power. During operation with the Atkinson cycle, the intake valve timing may be suitably adjusted to provide a lower compressor ratio. A lower octane content fuel may be injected during engine operation with the Atkinson cycle. During engine operation with the Atkinson cycle, the battery of the electric motor may be charged. If a higher than threshold power is desired, the intake valve timing may be adjusted to operate the engine with an Otto cycle. While operating with the Otto cycle, the engine may operate with a higher compression ratio to deliver the higher power output. In order to further facilitate engine operation with the Otto cycle, an octane booster (additive) may be injected to the fuel system to increase the octane content in the fuel supplied to the cylinders for combustion. The amount of octane booster injected may be based on the current octane level relative to the desired power level. Also, spark timing may be adjusted to operate the engine with an Otto cycle instead of an Atkinson cycle.
In this way, by opportunistically injecting an octane booster to the fuel system and adjusting intake valve timing, an engine may be operated with an Otto cycle to meet a higher torque demand during hybrid vehicle operation. By operating the engine with an Atkinson cycle during conditions when lower power output is desired, and/or when a battery state of charge is lower, fuel efficiency may be improved, and the battery may be charged. The technical effect of adding an octane booster to the injected fuel during engine operation with an Otto cycle is that the octane content of the fuel may be adjusted while the engine is operated with the Otto cycle, allowing for a higher engine output with improved fuel efficiency. Overall, by opportunistically operating a vehicle engine with an Atkinson cycle and an Otto cycle, fuel efficiency may be improved, and desired power output may be delivered.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.